Field of the Invention
The present invention relates to an optical system, which is suitable for being used as an image pickup optical system to be used for an image pickup apparatus such as a silver-halide film camera, a digital still camera, a digital video camera, a monitoring camera, and a TV camera.
Description of the Related Art
In recent years, there has been a demand for an optical system used for an image pickup apparatus to be compact as a whole and to have a wide field angle that facilitates photographing of a wide range. For example, a video camera, an on-vehicle camera, and the like are demanded to have as wide a field angle as a photographing field angle of at least 120° in order to photograph a wide range and to have a small distortion in order to enhance visibility in the periphery of a screen. Hitherto, as an optical system that meets those demands, there is known an optical system including a front group having a negative refractive power, an aperture stop, and a rear group having a positive refractive power in order from an object side to an image side.
In Japanese Patent Application Laid-Open No. 2004-145256, there is disclosed a wide angle lens including a negative lens, a cemented lens of a positive lens and a negative lens, an aperture stop, a positive lens, and a cemented lens of a positive lens and a negative lens in order from the object side to the image side and having a photographing field angle of from 130° to 180°. In Japanese Patent Application Laid-Open No. H04-267212, there is disclosed a super wide angle lens including a negative lens, a negative lens, a positive lens, a positive lens, and a cemented lens of a negative lens and a positive lens in order from the object side to the image side and having a photographing field angle of 140°.
In the optical system having a wide field angle, in general, the front group having a negative refractive power is arranged on the object side relative to the aperture stop, and the rear group having a positive refractive power is arranged on the image side relative thereto. This arrangement exhibits an asymmetric refractive power across the aperture stop, which is liable to cause various aberrations ascribable to asymmetry such as a coma, a distortion, and a lateral chromatic aberration. Particularly when an attempt is made to shorten an entire lens length while achieving a wide field angle, the refractive power of the negative lens on the object side becomes strong, and the refractive power of the positive lens on the image side becomes strong, which makes it difficult to maintain the distortion to a low level.
In Japanese Patent Application Laid-Open No. 2004-145256, the cemented lens having a positive refractive power is arranged closest to the image side. In addition, the distortion is set to from −48% to −85% by a central projection method with an ideal image height of Y=f×tan(ω), where Y represents an image height, f represents a focal length of the entire system, and ω represents a half field angle, to thereby reduce the distortion while achieving a wide field angle.
Further, in Japanese Patent Application Laid-Open No. H04-267212, the cemented lens having a positive refractive power is arranged closest to the image side. In addition, the distortion, is set to −70% by the central projection method with the ideal image height of Y=f×tan( 63 ), to thereby reduce the distortion while achieving a wide field angle. However, when an attempt is made to shorten the entire lens length while maintaining the wide field angle, the refractive power of the negative lens close to the object side and the refractive power of the positive lens close to the image side become strong, which increases the distortion.
In order to obtain high optical characteristics with a wide field angle and a small distortion over the entire screen while shortening the entire lens length and downsizing the entire system, it is important to appropriately set a lens configuration of lens units before and after the aperture stop within the optical system. Without appropriate settings thereof, it is difficult to obtain a compact optical system having high optical characteristics with a wide field angle and a small distortion.